The IS2000 standard (release-A or 1xRTT) supports up to 153.6 kbps peak user data rate, based on the CDG (CDMA Development Group) requirements for cdmaOne family evolution. One known so-called HDR (High Data Rate) system represents a major step forward to improve the packet data system throughput. A further improved version, HDR-1xEV-DO, has been adopted as an IS-856 standard, which aims to meet the CDG requirement for 1xRTT and evolution thereof. IS-856 is a data only system which will be overlaid on the 1xRTT network and uses a single carrier of 1.25 MHz co achieve a similar data rate (2 Mbps) to a 3-carriers configuration of CDMA2000. Although the chip rate is same as in IS-95/1xRTT, i.e. 1.2288 Mcps, 1xEV-DO introduces multiple enabling technologies. The basic 1xEV-DO employs a multi-code of Walsh sequence to construct a fat data pipe. However, such a multi-code channel with low spreading factor suffers from orthogonality loss in the dispersive channel due to inter-code interference, i.e., inter-symbol interference.
There are several key differences between IS-856 and traditional CDMA systems. For example, IS-856 is intended for non-real-time packet data. IS-856 is also a pure TDMA (Time Division Multiple Access) system, although signals are whitened by a scrambling code before transmission, occupies 1.25 MHz spectrum, and uses QPSK (Quadrature Phase Shift Keying), 8-PSK and 16-QAM (Quadrature Amplitude Modulation) modulation, only turbo codes, and full power transmission. Rate adaptation with a peak race of 2.48 Mbps, mobility support, proportional fairness scheduling, multi-user diversity, and connection with a PDSN (Packet Data Serving Node) and then IP (Internet Protocol), and therefore no connection with an MSC (Mobile Switching Center) or a circuit switched core network, represent further characteristics by which IS-856 differs from traditional CDMA systems.
The spectrum efficiency of IS-856 is achieved mainly by its TDMA signal structure along with scrambling codes, which makes it possible for frequency re-use one, fat-pipe scheduling to collect multi-user diversity by closely tracking the Rayleigh fading channel, high order modulation, and use of turbo codes.
Similar to CDMA2000, the downlink physical channel of 1xEV-DO occupies a 1.25 MHz spectrum with a chip rate 1.2288 Mcps. However, there is only one type of physical channel, which is divided into frames of 32768 chips or 26.67 ms. Each frame is further divided into 16 slots each has a length of 2048 chips or 1.67 ms. The slot is the basic unit and all other channels, such as the pilot channel, MAC (Media Access Control) channel, and traffic channels, will be multiplexed into a slot. In the downlink transmission direction, slots are classified into two modes, including active and idle. In active mode, an access network has either control information or user traffic information to send along with the pilot and MAC channels multiplexed into that slot. In idle mode, the access network transmits only the pilot and MAC channels. Each half slot has 96 pilot chips and 2*64 MAC chips. In summary, the pilot channel utilizes 9.375% of bandwidth and the MAC channel occupies 12.5% bandwidth.
One current 1xEV-DO terminal receiver chip solution is based on the rake receiver structure. The rake receiver works quite well in multi-path environments when the spreading factor is larger than 16. However, in order to increase the data throughput, 1xEV-DO employs a multi-code downlink with spreading factor 16. In this case, the rake receiver-based 1xEV/DO receiver has several limitations for implementation high level modulations. First, the baseband filter used at a transmitter is the same as in CDMA2000/IS-95, i.e. a 48-tap (chip) non-Nyquist filter, which always causes ICI (inter-chip interference). In addition, the maximum number of rake receiver fingers is 4, which sets a CIR (Carrier to Interference Ratio) ceiling of 17.8 dB. The relatively small bit width per sample further limits receiver performance.
All of these limitations may dramatically degrade the system performance when the multipath environment is rich, such as in a dense urban environment. The low sampling rate also causes finger detection inaccuracy. When the speed of a mobile communication device is high, tracking of channel variation may also be difficult. In practice, high level modulation such as 16-QAM cannot be used due to ICI caused by non-Nyquist filtering, and the 1xEV-DO high throughput is thereby generally limited to that achieved by Turbo coding and fast scheduling.